Authors: A. Montoya, J.J. Manclús, A. Arnau, Y. Jiménez, J.V. García-Narbón, C. March

Event:  8^th MGPR International Symposium of Pesticides in Food and the Environment in Mediterranean Countries. Cappadocia,  Turkey (2013)

The detection of very small mass changes by means of the Quartz Crystal Microbalance (QCM) is a very popular transduction technique in biosensor design for analytical applications. In fact, QCM biosensors for pesticide analysis in fruit and derived products have recently been reported. Despite their interesting analytical performance, these biosensors still need a thorough optimization of several essential analytical parameters. In particular, sensitivity and limit of detection should be significantly improved, so that pesticide biosensors could extend their applicability to more demanding applications, such as the analysis of drinking water. In order to overcome these limitations, a new concept of piezoelectric biosensor is proposed. Unlike classic QCM, where frequency changes are detected as the response to small mass changes, the new biosensor is based on the detection of phase changes, working at high constant frequency (high fundamental frequency: HFF). On this basis, a new 100 MHz QCM immunosensor based on monoclonal antibodies for pesticide analysis has been developed. A completely new device including a dedicated PEEK chip support to attach the HFF functionalized crystals, the automatic injection system, and the electronic characterization system, has been designed and built. Carbaryl and Thiabendazole were chosen as model pesticides to assess the immunosensor performance. Specific monoclonal antibodies were used as biorecognition molecules, working in competitive immunoassays in the conjugate-coated format. Specific hapten conjugates were covalently attached to the gold surfaces of the sensor electrodes by alkanethiol self-assembled monolayers (SAM). As compared to the previous low-frequency QCM immunosensors, the sensitivity was improved by more than a one order of magnitude (I50 values in the sub-µg I^-1 range), whereas the limits of detection improved by around two orders of magnitude (LODs around 0.16 µg I^-1). Immunoregent consumption was decreased by 5 times (antibody) and 1000 times (assay conjugate). The high sensitivity reached by these immunosensors allows pesticide analysis at concentrations near the European MRLs for drinking water.

Authors: J.V. García, C. March, A. Sánchez, Y. Montagut, Y. Jiménez, A. Montoya, A. Arnau

Event: Biosensors 2012, 22 World Congress on Biosensors. Cancún, México (2012)

A 50 MHz quartz crystal microbalance (QCM) immunosensor has been developed for the determination of pesticides. High fundamental frequency (HFF) inverted mesa quartz crystal resonators were used. To provide mechanical stability and robustness quartz crystal resonators were attached permanently to a PEEK chip holder. This also allowed easy manipulation in all experimental processes. An automated flow injection analysis system was developed. The system consists of: the flow cell for the sensor chips; a flow circuit with automated syringe pumps, distribution and injection valves; and an electronic characterization system, based on the phase mass measurement at constant frequency. Carbaryl pesticide was chosen as a model analyte. Due to the low molecular weight of this insecticide, the immunoassay format chosen was a competitive one, with monoclonal antibodies (MAbs). The hapten conjugate was covalently immobilized, via mercaptohexadecanoic acid (MHA) self assembled monolayer (SAM), onto the sensor electrode surface. The immunosensor results showed an improvement of one order of magnitude in terms of sensitivity (I₅₀ ≈ 2 μg/L) and two orders of magnitude in terms of limit of detection (LOD < 0,2 μg/L), compared with results obtained with standard 10MHz QCM sensors previously reported by our group. The automated analysis system allowed a very good reproducibility of measurements. Further improvement in terms of sensitivity and limit of detection could be easily accomplished working with higher fundamental frequency sensors (100-150 MHz), which can be directly used with the developed system.

Authors: J. Fochtmann, C. Peters, R. Fernández, R. Lucklum, D. McCann, J. Vetelino, A. Arnau

Journal: Sensors and actuators B: Chemical (2012)

Investigating the electrical sensitivity of lateral field excited resonators (LFE) we observed two characteristics to highly impact sensitivity. First, an increase of the piezoelectric coupling factor k enhances the viscosity measurement range. Higher coupling factors are provided by the use of new materials, namely lithium tantalite (LiTaO3) and lithium niobate (LiNbO3) with coupling factors of 0.43 and 0.91 respectively. Since the sensitivity to electrical load parameters is governed by relative permittivity of the resonator material itself, the sensitivity to electrical parameters is found to decrease for new materials. Second, an electrode pattern that reveals higher energy trapping in the center of crystal was investigated. Compared to common LFE pattern, a third electrode was introduced in center between bite wing electrodes. Finite element modeling in ANSYS© shows enhanced sensitivity to low relative permittivity in range from 1 to 8. We performed experiments to verify the simulation results. Although an increased frequency shift compared to common LFE resonators was not observed, significantly higher conductance amplitudes occurred, which enhances the measurement range to high viscosity liquids, too.

Authors: Y. Montagut, J.V. García, Y. Jiménez, C. March, A. Montoya, A. Arnau

Journal: Review of Scientific Instruments (2011)

The improvement of sensitivity in quartz crystal microbalance (QCM) applications has been addressed in the last decades by increasing the sensor fundamental frequency, following the increment of the frequency/mass sensitivity with the square of frequency predicted by Sauerbrey. However, this sensitivity improvement has not been completely transferred in terms of resolution. The decrease of frequency stability due to the increase of the phase noise, particularly in oscillators, made impossible to reach the expected resolution. A new concept of sensor characterization at constant frequency has been recently proposed. The validation of the new concept is presented in this work. An immunosensor application for the detection of a low molecular weight contaminant, the insecticide carbaryl, has been chosen for the validation. An, in principle, improved version of a balanced-bridge oscillator is validated for its use in liquids, and applied for the frequency shift characterization of the QCM immunosensor application. The classical frequency shift characterization is compared with the new phase-shift characterization concept and system proposed.